The long term goals of this research are to identify the interneurons which mediate the primary afferent depolarization (PAD) of muscle and cutaneous afferent fibers in the cat spinal cord, and to define more precisely the reflex pathways to which they belong, as well as the supraspinal influences which modulate their activity. Experiments will be performed in anesthetized, decerebrated and decerebrated/spinal cats, either spontaneously or paralyzed and under artificial respiration. Particular attention will be given to: a), characterization of the intracellular PAD patterns produced by segmental and descending inputs on physiologically identified single group Ia, Ib and II afferent fibers left in continuity with the medial gastrocnemius (MG) muscle, both in normal preparations and between 1 week and 9 months after sectioning and resuturing the MG nerve. b), to determine if the excitatory and inhibitory actions that cutaneous fibers have on the PAD of Ib fibers are modulated by supraspinal tonic descending influences. c), to characterize the possible correlation of the DC potential changes and potassium transients with the PAD of cutaneous fibers produced by stimulation of supraspinal nuclei, and the effects of drugs blocking GABAergic and serotonergic transmission on the PAD. d), to investigate the actions of single intermediate nucleus interneurons on the motoneuron synaptic potentials and on dorsal root potentials elicited by group 1 and by cutaneous volleys, e), to disclose the transmitters possibly involved in the inhibitory actions of intermediate nucleus interneurons on motoneurons and f), analyze the fluctuations of monosynaptic EPSPs elicited by single la fibers in spinal MG motoneurons during various amounts of stretch of synergistic and antagonistic muscles and during electrical stimulation of motor nerves producing depression of la-EPSPs.